Microwave stripline variable attenuator having compressible,lossy dielectric material



prll 7, 1970 Q B|5||Op 3,505,619

MICROWAVEASTRIPLINE VARIABLE ATTENUATOR HAVING coMPREssIBLE, LossY DIELECTRIC MATERIAL Filed 00T.. 17, 1968 IUnited States Patent O 3,505,619 MICROWAVE STRIPLINE VARIABLE ATTENUA- TOR HAVING COMPRESSIBLE, LOSSY DIELEC- TRIC MATERIAL Otis L. Bishop, Raleigh, N.C., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Oct. 17, 1968, Ser. No. 768,452 Int. Cl. H01p 1/22 U.S. Cl. 333-81 10 Claims ABSTRACT OF THE DISCLOSURE A variable attenuator used in microwave strip transmission lines wherein the variability of the attenuator is achieved by the compression of a lossy resilient dielectric material inserted in one of two dielectric sheets located between one of a pair of ground plane conductors and a relatively thin center conductor strip attached to the other dielectric sheet.

BACKGROUND OF THE INVENTION Transmission lines for electromagnetic energy in the microwave region normally use coaxial or waveguide conductors. More recently, however, in large integrated systems where weight, small size, and lower fabrication costs are a significant factor, the microwave strip transmission line has gained wide acceptance and use. Such stripline apparatus is comprised of a thin strip center conductor separated and mechanically supported from a single or double ground plane conductor by solid dielectric material. When desirable, the center conductor can be etched on one or both sides of a dielectric sheet which is supported by posts between the ground plane conductors. These striplines are usually designed to operate in the TEM mode. This requires that two ground planes be employed and that the spacing between the ground planes be less than one-half Wavelength if higher order modes are to be suppressed. These devices are shown and described in greater detail, for example, in the publication entitled Electronic Designers Handbook, McGraw-Hill Book Company, Inc., 1957, at pages -42 through 20-44.

Level set attenuators are often used in microwave circuits to correct for changes in generator output power or to optimize circuit elements such as mixers. In most of these circuits the level setattenuator is required to be only slightly variable around a predetermined median attenuation and there is usually no requirement on precision or calibration. Although stripline variable attenuators are also known to those presently skilled in the art, the configurations utilize the concept of sliding one of the line elements with respect to another. These attenuators, however, are large and require relatively sophisticated mechanical systems. As a result, designers of stripline apparatus most often set the power level with external coaxial or waveguide variable attenuators, preferring to delete the attenuator where possible.

SUMMARY OF THE INVENTION The subject invention is directed to an improved microwave strip transmission line attenuator which is variable in accordance with a simple mechanical system for varying the attenuation and comprises a pair of conductive ground planes; at least two dielectric sheets interposed between the ground planes; a relatively thin electrically conductive center strip attached to one of the dielectric sheets on the surface away from the ground planes; a region of lossy, resilient dielectric material located in a separation of predetermined dimensions in the other dielectric sheet, said separation being located along the 3,505,619 Patented Apr. 7, 1970 ICC BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a fragmentary central cross-section view of the preferred embodiment of the subject invention; and

FIGURE 2 is a top plan view of the preferred ernbodiment shown in FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Directing attention now to FIGURE l, the present invention is comprised of a microwave strip transmission line operating in the TEM mode and comprises, inter alia, a pair of electrically conductive ground planes 10 and 12 separated by two sheets of dielectric material 14 and 16. A single strip of conductive material having a relatively thin width and which may be, for example, copper is bonded to one of the dielectric sheets 16 so that it runs centrally along its length. Additionally, the thin conductive strip 18, hereinafter referred to as the center conductor, is separated from the ground plane 12 by the thickness of the dielectric sheet 16. Since the thickness of the other sheet of dielectric material 14 is of substantially the same thickness, the center conductor 18 is located substantially midway between the pair of ground planes 10 and 12. A substantially rectangular portion 20 (shown in FIG. 2) of the dielectric sheet 14 is removed providing a discontinuity thereby and is replaced by aregion of lossy resilient dielectric material 22 in the form of a polystyrene'foam having substantially the same thickness as the dielectric sheet 14. A thin electrically conductive leaf spring 24 is located on the underside of the conductive ground plane 10 next to the upper surface of the resilient dielectric material 22 and is attached to the ground plane 10 by its ends 26 and 28. The remainder of the leaf spring 24 is capable of moving freely. The leaf spring 24 moreover is adapted to be urged against the lossy resilient dielectric material 22 by means of a set screw 30 inserted through the conductive ground plane 10 so that it is centrally located with respect to the surface of the leaf spring 24.

The lossy resilient dielectric foam 2 2 is a part of the stripline circuit at all times. The relative dielectric constant of the lossy resilient foam is chosen to have a magnitude substantially that of the dielectric sheets in the adjacent stripline. This minimizes the dielectric discontinuity at the interfaces. Further, the lossy material reduces the discontinuity by attenuating reections from within. The entire apparatus has a minimum insertion loss determined by the loss tangent and length of the lossy resilient dielectric material.

The variability of the attenuator shown in FIGURES l and 2 is obtained by compressing the foam material 22 by means ofthe spring 24 and the set screw 30. The compression force is applied in the middle of the leaf spring 24 so that the middle of the lossy dielectric resilient foam 22 undergoes the greatest compression corresponding to the midpoint of its length and wherein the thickness is then tapered to its outer edges. As the lossy resilient dielectric foam 22 is compressed, the relative dielectric constant of loss per unit length increases while a decrease occurs in the spacing from the center conductor 18 to the ground plane 10 since the leaf spring 24 also forms part of the ground plane 10. The increase in dielectric constant and decrease in the center conductor to ground plane spacing causes the electric fields to concentrate on the side of the center conductor 18 next to the lossy resilient foam member 22 and is thus further attenuated by the increased loss per unit length. The taper of the cross-sectional thickness of the lossy resilient foam 22 caused by the action of the set screw 30 and the leaf spring 24 helps to maintain a circuit match in the apparatus.

Although it is to be understood that the following is not meant to be interpreted in a limiting sense, a variable attenuator has been constructed utilizing chemically crosslinked polystyrene dielectric sheets of 1/16" thickness and a 3A" length of lossy, resilient polystyrene foam sold commercially by Emerson & Cuming, Inc. as LS22. The following characteristics were exhibited: in the X-band range of microwave frequencies a minimum attenuation of 5 db and a maximum attenuation of 12 db was obtained with a deviation of r0.5 db. The maximum VSWR was less than 1.55 which occurred at the minimum attenuator setting. Normal use, however, would require a median setting.

In summation, the present invention relates to a microwave stripline level set attenuator requiring only a relatively simple mechanical system for varying the attenuation. By means of compressing a resilient dielectric material inserted in a region wherein a dielectric sheet separates the ground plane from the center conductor, an attenuator having a low reflection coefiicient is achieved. By selective compression of the resilient dielectric an attenuation of greater than twice the initial value expressed in decibels is obtainable. The simplicity of the mechanical system, moreover, results in small size and easy integration into large stripline circuits.

While there has been shown and described what is at present considered to be a preferred embodiment of the invention, modifications thereto will readily occur to those skilled in the art. For this reason, it is not desired that the invention be limited to the specific arrangement shown and described but it is to be understood that all equivalents, alterations and modifications within the spirit and scope of the present invention are herein meant to be included.

I claim:

1. A varia-ble microwave strip transmission line attenuator comprising in combination:

electrically conductive ground plane means;

dielectric means located next to said ground plane means and -additionally including a discontinuity thereof having predetermined dimensions in a selected portion of said dielectric means;

an electrically conductive center conductor strip attached to said dielectric means being uniformly spaced away from said conductive ground plane means thereby;

a region of relatively lossy, resilient dielectric material located in said discontinuity, being contiguous on one surface thereof with said electrical conductive strip;

metallic spring means connected to said ground plane means and being contiguous with the opposite surface of said lossy, resilient dielectric material;

and mechanical means coupled to said spring means for selectively varying the cross-sectional area of the lossy, resilient dielectric material by movement of said spring means.

2. The invention as defined by claim 1 wherein said discontinuity comprises a quadrangle having a dimension transverse to said center conductor strip which is substantially greater than the width of said center conductor strip.

3. The invention as defined by claim 1 wherein said dielectric `means comprises a first and a second dielectric sheet located side-by-side and wherein said discontinuity is located in said first dielectric sheet and wherein said center conductor strip is attached to said second dielectric sheet.

4. The invention as defined by claim 3 wherein said spring means comprises a relatively thin leaf spring mounted coplanar with a surface of said conductive ground plane means contiguous with said first dielectric sheet and positioned against the surface of said lossy, resilient dielectric material.

5. The invention as defined by claim 4 and wherein said electrically conductive ground plane means recited in claim 1 comprises a first and a second ground plane each having a width dimension substantially greater than the width dimension of said center conductor strip and wherein said first conductive ground plane is contiguous with said irst sheet of dielectric material and wherein said second conductive ground plane is contiguous with said second dielectric sheet.

6. The invention as defined by claim 5 wherein said thin leaf spring recited in claim 4 has its ends attached to said first ground .plane so as to be pivotally movable about said ends and wherein said mechanical means comprises means for contacting said thin leaf spring intermediate said ends.

7. The invention as defined by claim 6 wherein said mechanical means comprises screw means mounted through said first conductive ground plane for varying the position of said thin leaf spring whereby said lossy, resilient material in contact there-with is adapted to be compressed.

8. The invention as defined lby claim 7 wherein said screw means is located in said first conductive ground plane substantially at the center of said thin leaf spring.

9. The invention as defined by claim 8 wherein said lossy, resilient dielectric material comprises a lossy, resilient polystyrene foam.

10. The invention as defined by claim 9 wherein said lossy, resilient dielectric foam has adielectric constant in its uncompressed state of a magnitude in the region of the dielectric constant of the first and second dielectric sheets and wherein the dielectric constant and loss per unit length of said dielectric foam increases in relation to the degree of compression provided by said leaf spring.

References Cited UNITED STATES PATENTS 10/ 1957 Engelmann. 6/ 1959 Arditi et al.

U.S. Cl. X.R. 3 33-84 

